Initialize a `uaparser.Parser` instance at the first UserAgent call and
reuse it instead of creating a new parser for every invocation.

Co-authored-by: Tyler Helmuth <12352919+TylerHelmuth@users.noreply.github.com>

…arsing

This reverts commit 6ac23c0. going for
a simpler solution where the useragent parser is instantiated just
before returning the anonymous ottl.ExprFunc[K] that uses it.
This will prevent re-instantating the parser at each `UserAgent(...)`
invocation.